Power storage device

ABSTRACT

A power storage device with reduced initial irreversible capacity is provided. The power storage device includes a positive electrode including a positive electrode current collector and a positive electrode active material layer, a negative electrode including a negative electrode current collector and a negative electrode active material layer, and an electrolyte solution, in the negative electrode active material layer, the content percentage of a carbon material with an R value of 1.1 or more is less than 2 wt %, The R value refers to a ratio of a peak intensity I 1360  to a peak intensity I 1580  (I 1360 /I 1580 ). The peak intensity I 1360  and the peak intensity I 1580  are observed by Raman spectrometry at a Raman shift of 1360 cm −1  and a Raman shift of 1580 cm −1 , respectively. The electrolyte solution contains a lithium ion and an ionic liquid composed of an organic cation and an anion.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power storage device using an ionicliquid.

2. Description of the Related Art

Owing to an increase in demand for portable electronic devices such as amobile phone and a laptop personal computer and development of anelectric vehicle (EV) and the like, a demand fir power storage devicessuch as an electric double layer capacitor, a lithium ion secondarybattery, and a lithium ion capacitor has been significantly increasingin recent years. For the power storage devices, high capacity, highperformance, safety in various operating environments, and the like arerequired.

To satisfy the above requirements, electrolyte solutions for the powerstorage devices have been actively developed. Cyclic carbonates are usedfor the electrolyte solutions for the power storage devices. Inparticular, ethylene carbonate is often used because of its highdielectric constant and high ionic conductivity.

However, not only an ethylene carbonate but also many other organicsolvents have volatility and a low flash point. For this reason, in thecase of using an organic solvent for an electrolyte solution for a powerstorage device, the temperature inside the power storage device mightrise due to a short circuit, overcharge, or the like and the powerstorage device might burst or catch fire.

In consideration of the risks, the use of an ionic liquid, which isnonvolatile and flame-retardant, for an electrolyte solution of a powerstorage device has been studied. An ionic liquid is also referred to asa room temperature molten salt, which is a salt formed by thecombination of a cation and an anion. Examples of the ionic liquid arean ionic liquid including a quaternary ammonium-based cation and anionic liquid including an imidazolium-based cation (see Patent Document1 and Non-Patent Document 1).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2003-331918

Non-Patent Document

[Non-Patent Document 1] Hajime Matsumoto, et al., Fast cycling ofLi/LiCoO₂ cell with low-viscosity ionic liquids based onbis(fluorosulfonyl)imide [FSI]⁻, Journal of Power Sources 160, 2006, pp.1308-1313

SUMMARY OF THE INVENTION

In the case of a power storage device including a coated electrode usinga carbon material (e.g., spherical graphite such as mesocarbonmicrobeads), it is important to add a conductive additive to keep theelectric conductivity of the whole of the electrode high and uniform inorder to prolong the cycle life of the power storage device. As theconductive additive, preferably, a carbon material such as acetyleneblack (AB) or carbon black has been conventionally used.

When a charge and discharge test is conducted on a cell including acarbon material such as mesocarbon microbeads as a negative electrodeactive material and an electrolyte solution containing an organicsolvent, initial irreversible capacity is not influenced even when aseveral weight percent of acetylene black is contained as a conductiveadditive of the negative electrode in the cell.

However, when a several weight percent of acetylene black is containedas a conductive additive of a negative electrode in a cell including anelectrolyte solution containing an ionic liquid, depending on a cationspecies of the ionic liquid, the electrolyte solution containing theionic liquid reacts with the acetylene black at a potential differentfrom a potential at which a lithium insertion reaction occurs.

Examples of the reaction between the electrolyte solution containing theionic liquid and acetylene black are insertion of a cation species ofthe ionic liquid at a potential higher than a potential at which lithiumis inserted, and decomposition of the ionic liquid due to the reactionwith a surface of a negative electrode active material. Thus, theinitial irreversible capacity of a power storage device is increased,resulting in a problem of a decrease in initial capacity.

In view of the above problem, an object of one embodiment of the presentinvention is to provide a power storage device with reduced initialirreversible capacity.

One embodiment of the present invention is a power storage deviceincluding an electrolyte solution containing an ionic liquid and anegative electrode active material layer in which the content percentageof a less-graphitized carbon material is decreased.

A degree of graphitization is expressed by an R value that is the ratioof a peak intensity I₁₃₆₀ to a peak intensity I₁₅₈₀. The peak intensityI₁₃₆₀ refers to a peak intensity at a Raman shift of 1360 cm⁻³ of aRaman spectrum observed by Raman spectrometry and the peak intensityI₁₅₈₀ refers to a peak intensity at a Raman shift of 1580 cm⁻¹ of theRaman spectrum.

One embodiment of the present invention is a power storage deviceincluding a positive electrode which includes a positive electrodecurrent collector and a positive electrode active material layer, anegative electrode which includes a negative electrode current collectorand a negative electrode active material layer, and an electrolytesolution. In the negative electrode active material layer, the contentpercentage of a carbon material with an R value of 1.1 or more is lessthan 2 wt %. The R value refers to a ratio of a peak intensity I₁₃₆₀ toa peak intensity I₁₃₈₀ (I₁₃₆₀/I₁₅₈₀). Here, the peak intensity I₁₃₆₀ andthe peak intensity I₁₅₈₀ are observed by Raman spectrometry at a Ramanshift of 1360 cm⁻¹ and a Raman shift of 1580 cm⁻¹, respectively. Theelectrolyte solution contains a lithium ion and an ionic liquid composedof an organic cation and an anion.

A decrease in the content percentage of the less-graphitized carbonmaterial contained in the negative electrode active material layer caninhibit an insertion of a cation species of the ionic liquid containedin the electrolyte solution at a potential higher than a potential atwhich lithium is inserted. In addition, the ionic liquid contained inthe electrolyte solution is inhibited from reacting with a surface of anegative electrode active material to be decomposed. Thus, the initialirreversible capacity can be reduced in the power storage device.

One embodiment of the present invention can provide a power storagedevice with reduced initial irreversible capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a cross-sectional view and an external view of apower storage device.

FIGS. 2A and 2B arc an external view and a cross-sectional view of apower storage device.

FIG. 3 is a diagram of application examples of a power storage device.

FIGS. 4A to 4C are diagrams of an application example of a power storagedevice.

FIGS. 5A and 5B are diagrams of an application example of a powerstorage device.

FIGS. 6A and 6B show results of a charge and discharge test.

FIGS. 7A and 7B show results of a charge and discharge test.

FIGS. 8A and 8B show results of a charge and discharge test.

FIGS. 9A and 9B show cyclic voltammnograms.

FIG. 10 shows the Raman spectrum of acetylene black.

FIG. 11 shows the Raman spectrum of mesocarbon microbeads.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below in detailwith reference to the accompanying drawings. Note that the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that a variety of changes andmodifications can be made without departing from the spirit and scope ofthe present invention. Therefore, the present invention is not construedas being limited to the description of the embodiments and examplesgiven below.

Embodiment 1

In this embodiment, a structure and a method for manufacturing a powerstorage device of one embodiment of the present invention is describedwith reference to FIGS. 1A and 1B and FIGS. 2A and 2B.

A power storage device in this specification and the like refers to anyelement having a function of storing power and any device having afunction of storing power. For example, a lithium ion secondary battery,a lithium ion capacitor, and an electric double layer capacitor areincluded in the category of the power storage device.

FIG. 1A illustrates a laminated lithium ion secondary battery as anexample of the power storage device.

A laminated lithium ion secondary battery 100 in FIG. 1A includes apositive electrode 103 including a positive electrode current collector101 and a positive electrode active material layer 102, a negativeelectrode 106 including a negative electrode current collector 104 and anegative electrode active material layer 105, a separator 107, anelectrolyte solution 108, and a housing 109. The separator 107 is placedbetween the positive electrode 103 and the negative electrode 106provided in the housing 109. The housing 109 is filled with theelectrolyte solution 108.

First, the structure of the negative electrode 106 is described.

For the negative electrode current collector 104, it is possible to usea highly conductive material, for example, a metal such as copper,nickel, or titanium. The negative electrode current collector 104 canhave a foil-like shape, a plate-like shape (sheet-like shape), anet-like shape, a punching-metal shape, an expanded-metal shape, or thelike, as appropriate. The negative electrode current collector 104preferably has a thickness of more than or equal to 10 μm and less thanor equal to 30 μm.

The negative electrode active material layer 105 contains a negativeelectrode active material. An active material refers only to a materialwhich relates to insertion and extraction of an ion that is a carrier.In this specification and the like, a material that is actually a“negative electrode active material”, a conductive additive, a binder,and the like are collectively referred to as a negative electrode activematerial layer.

In one embodiment of the present invention, the negative electrodeactive material layer 105 contains a highly graphitized carbon material.A degree of graphitization is expressed by an R value that is the ratioof a peak intensity I₁₃₆₀ to a peak intensity I₁₅₈₀. The peak intensityI₁₃₆₀ refers to a peak intensity of a Raman spectrum observed by Ramanspectrometry at a Raman shift of 1360 cm⁻¹ and the peak intensity I₁₅₈₀refers to a peak intensity of the Raman spectrum at a Raman shift of1580 cm⁻¹. The smaller the R value is, the higher the degree ofgraphitization is. For this reason, in one embodiment of the presentinvention, a carbon material with an R value of less than 1.1,preferably 0.3 or less is used in the negative electrode active materiallayer 105.

In the case where an ionic liquid is contained in the electrolytesolution included in the power storage device, when a several weightpercent of less-graphitized material, for example, acetylene black orthe like with an R value of more than 1.1 is contained in the negativeelectrode active material layer 105, the electrolyte solution containingthe ionic liquid reacts with the acetylene black at a potentialdifferent from a potential at which a lithium insertion reaction occurs.Examples of the reaction between the electrolyte solution containing theionic liquid and acetylene black are insertion of a cation species ofthe ionic liquid at a potential higher than a potential at which lithiumis inserted, and decomposition of the ionic liquid due to the reactionbetween the ionic liquid and a surface of the negative electrode activematerial. This might lead to an increase in the initial irreversiblecapacity of the power storage device.

In view of the above, in one embodiment of the present invention, thecontent percentage of a carbon material with an R value of 1.1 or morein the negative electrode active material layer is less than 2 wt %. TheR value refers to a ratio of a peak intensity I₁₃₆₀ to a peak intensityI₁₅₈₀ (I₁₃₆₀/I₁₅₈₀). Here, the peak intensity I₁₃₆₀ and the peakintensity I₁₅₈₀ are observed by Raman spectrometry at a Raman shift of1360 cm⁻¹ and a Raman shift of 1580 cm⁻¹, respectively.

By reducing the content percentage of the less-graphitized carbonmaterial (the R value of which is 1.1 or more) in the negative electrodeactive material layer 105 (by setting the content percentage to lessthan 2 wt %, preferably zero), the cation species of the ionic liquidcan be inhibited from being inserted into the less-graphitized carbonmaterial. In addition, the ionic liquid can be inhibited from reactingwith a surface of the negative electrode active material to bedecomposed. Thus, the initial irreversible capacity of the power storagedevice can be reduced.

In one embodiment of the present invention, as the highly graphitizedcarbon material (the R value of which is less than 1.1, preferably 0.3or less) contained in the negative electrode active material layer 105,for example, natural graphite, artificial graphite such as mesocarbonmicrobeads, mesophase picth-based carbon fibers, isotropic pitch-basedcarbon fibers, or graphene can be used.

A particle of the carbon material is preferably a scaly particle, anagglomerate particle, a fiber particle, a whisker particle, a sphericalparticle, a flaky particle, or a granular particle.

The above-described carbon materials can each function as either anegative electrode active material or a conductive additive. Thus, oneor more of the carbon materials may be contained in the negativeelectrode active material layer 105. Note that the conductive additivedesirably increases the points and the area where the negative electrodeactive materials are in contact with each other.

Flaky graphene has an excellent electrical characteristic of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength. For this reason, the use of graphene as theconductive additive can increase the points and the area where thenegative electrode active materials are in contact with each other.

As the conductive additive, a material other than the less-graphitizedcarbon material can be used. Metal powder or metal fibers of copper,nickel, aluminum, silver, gold, or the like, a conductive ceramicmaterial, or the like can be used.

Note that graphene in this specification includes single-layer grapheneand multilayer graphene including two to hundred layers. Single-layergraphene refers to a sheet of one atomic layer of carbon moleculeshaving π bonds. Graphene oxide refers to a compound formed by oxidationof such graphene. When graphene oxide is reduced to form graphene,oxygen contained in the graphene oxide is not entirely extracted andpart of the oxygen remains in the graphene. In the case where graphenecontains oxygen, the proportion of the oxygen measured by XPS is higherthan or equal to 2% and lower than or equal to 20%, preferably higherthan or equal to 3% and lower than or equal to 15%.

As a hinder, a polysaccharide such as starch, carboxymethyl cellulose,hydroxypropyl cellulose, regenerated cellulose, or diacetyl cellulose; avinyl polymer such as polyvinyl chloride, polyethylene, polypropylene,polyvinyl alcohol, polyvinyl pyrrolidone, polytetrafluoroethylene,polyvinylidene fluoride (PVDF), ethylene-propylene-diene monomer (EPDM)rubber, sulfonated EPDM rubber, styrene-butadiene rubber, butadienerubber, and fluorine rubber; polyether such as polyethylene oxide; orthe like can be used.

Next, a method for forming the negative electrode 106 using theabove-described carbon material is described.

In order to form the negative electrode active material layer 105,first, a negative electrode paste is formed. The negative electrodepaste can be formed in such a manner that the above-described carbonmaterial to which the conductive additive and the binder are added ismixed with an organic solvent. As the organic solvent,N-methylpyrrolidone (NMP) can be used, for example.

The negative electrode current collector 104 may be subjected to surfacetreatment. Examples of such surface treatment are corona dischargetreatment, plasma treatment, and undercoat treatment. The surfacetreatment increases the wettability the negative electrode currentcollector 104 to the negative electrode paste. In addition, the adhesionbetween the negative electrode current collector 104 and the negativeelectrode active material layer 105 can be increased.

Next, the negative electrode paste is applied on to the negativeelectrode current collector 104.

Then, the negative electrode paste is dried to form the negativeelectrode active material layer 105. In the drying step of the negativeelectrode paste, draught drying is performed at 70° C. in an airatmosphere for 30 minutes and then, drying is performed at 170° C. in areduced pressure environment for 10 hours. The negative electrode activematerial layer 105 formed in such a manner has a thickness of more thanor equal to 20 μm and less than or equal to 150 μm.

Note that the negative electrode active material layer 105 may bepredoped. A method for predoping the negative electrode active materiallayer 105 is not particularly limited. For example, the negativeelectrode active material layer 105 may be predoped electrochemically.For example, before the battery is fabricated, the negative electrodeactive material layer 105 can be predoped with lithium in an electrolytesolution described later with the use of a lithium metal as a counterelectrode.

Next, the structure of the positive electrode 103 is described.

As the positive electrode current collector 101, it is possible to use amaterial having high conductivity such as a metal typified by stainlesssteel, gold, platinum, zinc, iron, copper, aluminum, or titanium, or analloy thereof. For example, gold, platinum, aluminum, or the like ispreferable. Further, the positive electrode current collector 101 can beformed using an aluminum alloy to which an element which improves heatresistance, such as silicon, titanium, neodymium, scandium, ormolybdenum, is added. Further alternatively, the positive electrodecurrent collector 101 may be formed using a metal element which formssilicide by reacting with silicon. Examples of the metal element whichforms silicide by reacting with silicon are zirconium, titanium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,cobalt, and nickel. The positive electrode current collector 101 canhave a foil shape, a plate shape (sheet shape), a net shape, apunching-metal shape, an expanded-metal shape, or the like asappropriate. The positive electrode current collector 101 preferably hasa thickness of greater than or equal to 10 μm and less than or equal to30 μm.

The positive electrode active material layer 102 contains a positiveelectrode active material. An active material refers only to a materialwhich relates to insertion and extraction of an ion that is a carrier.In this specification and the like, a material that is actually a“positive electrode active material”, a conductive additive, a binder,and the like are collectively referred to as a positive electrode activematerial layer.

As the positive electrode active material, a compound such as LiFeO₂,LiCoO₂, LiNiO₂, or LiMn₂O₄, or V₂O₅, Cr₂O₅, or MnO₂ can be used.

Further, an olivine-type lithium containing complex phosphate (LiMPO₄(general formula) (M is one or more of Fe(I), Mn(II), Co(II), andNi(II))) can be used. Typical examples of the general formula LiMPO₄ areLiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄,LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄,LiNi_(a)Mn_(b)PO₄ (a+b≦1, 0<a<1, and 0<b<1), LiFe_(c)Ni_(d)Co_(e)PO₄,LiFe_(c)Ni_(d)Mn_(e)PO₄, LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≦1, 0<c<1, 0<d<1,and 0<e<1), and LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≦1, 0<f<1, 0<g<1,0<h<1, and 0<i<1).

Further, a lithium-containing complex silicate such as Li(_(2-j))MSiO₄(general formula) (M is one or more of Fe(II), Mn(II) Co(II), and Ni(II)and 0≦j≦2) can be used. Typical examples of the general formulaLi(_(2-j))MSiO₄ are L(_(2-j))FeSiO₄, Li(_(2-j))NiSiO₄, Li(_(2-j))CoSiO₄,Li(_(2-j))MnSiO₄, Li(_(2-j))Fe_(k)Ni_(l)SiO₄,Li(_(2-j))Fe_(k)Co_(l)SiO₄, Li(_(2-j))Fe_(k)Mn_(i)SiO₄,Li(_(2-j))Ni_(k)Co_(l)SiO₄, Li(_(2-j))Ni_(k)Mn_(l)SiO₄ (k+l≦1, 0<k<1,and 0<l<1), Li(_(2-j))Fe_(m)Ni_(n)Co_(q)SiO₄,Li(_(2-j))Fe_(m)Ni_(n)Mn_(q)SiO₄, Li(_(2-j))Ni_(m)Co_(n)Mn_(q)SiO₄(m+n+q≦1, 0<m<1, 0<n<1, and 0<q<1), andLi(_(2-j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≦1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

In the case where carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, the following may be used as thepositive electrode active material: a compound, a lithium-containingcomplex phosphate, or a lithium-containing complex silicate which isobtained by substituting an alkali metal (e.g., sodium or potassium), analkaline-earth metal (e.g., calcium, strontium, barium, beryllium, ormagnesium) for lithium in the compound, the lithium-containing compositephosphate, or the lithium-containing composite silicate.

Moreover, a variety of additives such as a conductive additive and abinder can be used in the positive electrode active material layer 102.

Note that in addition to the above-described conductive additive for thenegative electrode active material layer 105, a less-graphitized carbonmaterial may be used as the conductive additive for the positiveelectrode active material layer 102. Examples of the less-graphitizedcarbon material are carbon black such as acetylene black and ketjenblack.

Next, a method for forming the positive electrode 103 is described.

In order to form the positive electrode active material layer 102,first, a positive electrode paste is formed. The positive electrodepaste can be formed in such a manner that the above-described positiveelectrode active material to which the conductive additive and thebinder are added is mixed with an organic solvent. As the organicsolvent, NMP can be used, for example.

The positive electrode current collector 101 may be subjected to surfacetreatment. Examples of such surface treatment are corona dischargetreatment, plasma treatment, and undercoat treatment. The surfacetreatment increases the wettability of the positive electrode currentcollector 101 to the positive electrode paste. In addition, the adhesionbetween the positive electrode current collector 101 and the positiveelectrode active material layer 102 can be increased.

Then, the positive electrode paste is dried to form the positiveelectrode active material layer 102. In the drying step of the positiveelectrode paste, drying is performed at 135° C. in a reduced pressureenvironment for 2 hours and then, drying is performed at 170° C. underreduced pressure for 10 hours. The positive electrode active materiallayer 102 formed in such a manner has a thickness of more than or equalto 20 μm and less than or equal to 150 μm.

The electrolyte solution 108 is composed of a nonaqueous solvent and anelectrolyte.

In one embodiment of the present invention, an ionic liquid is used asthe nonaqueous solvent. One solvent or a mixed solvent of a plurality ofionic liquids may be used as the ionic liquid. Further, as thenonaqueous solvent, an organic solvent may be used; one organic solventor a mixed solvent of a plurality of organic solvents may be used.Furthermore, a mixed solvent of an ionic liquid and an organic solventmay be used.

An ionic liquid is composed of a cation and an anion. The ionic liquidof one embodiment of the present invention includes an organic cationand an anion. Examples of the organic cation are aliphatic oniumcations, such as a quaternary ammonium cation, a tertiary sulfoniumcation, and a quaternary phosphonium cation, and aromatic cations, suchas an imidazolium cation and a pyridinium cation. Examples of the anionare a monovalent amide anion, a monovalent methide anion, afluorosulfonate anion, a perfluoroalkylsulforiate anion,tetrafluoroborate, perfluoroalkylborate, hexafluorophosphate, andperfluoroalkylphosphate.

As the ionic liquid, an ionic liquid represented by General Formula (G1)shown below can be used, for example.

In General Formula (G1), R¹ to R⁶ separately represent an alkyl grouphaving 1 to 20 carbon atoms, a methoxy group, a methoxymethyl group, amethoxyethyl group, or a hydrogen atom.

Further, as the ionic liquid, an ionic liquid represented by GeneralFormula (G2) shown below can be used, for example.

In General Formula ((22), R⁷ to R¹³ separately represent an alkyl grouphaving 1 to 20 carbon atoms, a methoxy group, a methoxymethyl group, amethoxyethyl group, or a hydrogen atom.

Further, as the ionic liquid, an ionic liquid represented by GeneralFormula (G3) shown below can be used, for example.

In General Formula (G3), each of n and in is greater than or equal to 1and less than or equal to 3, and each of α and β is greater than orequal to 0 and less than or equal to 6. When n is 1, α is greater thanor equal to 0 and less than or equal to 4. When n is 2, α is greaterthan or equal to 0 and less than or equal to 5. When n is 3, α isgreater than or equal. to 0 and less than or equal to 6. When m is 1, βis greater than or equal to 0 and less than or equal to 4. When m is 2,β is greater than or equal to 0 and less than or equal to 5. When m is3, β is greater than or equal to 0 and less than or equal to 6. Notethat “α or β is 0” means that at least one of two aliphatic rings isunsubstituted. Note that the case where both α and β are 0 is excluded.X or Y is a substituent such as a straight chain or lateral chain alkylgroup having 1 to 4 carbon atoms, a straight chain or lateral chainalkoxy group having 1 to 4 carbon atoms, or a straight chain or lateralchain alkoxyalkyl group having 1 to 4 carbon atoms. Further, A⁻represents a monovalent amide anion, a monovalent methide anion, aperfluoroalkylsulfonate anion, tetrafluoroborate, perfluoroalkylborate,hexafluorophosphate, or perfluoroalkylphosphate.

Examples of the anion in General Formulae (G1) to (G3) are a monovalentamide anion, a monovalent methide anion, a fluorosulfonate anion(SO₃F⁻), a perfluoroalkylsulfonate anion, tetrafluoroborate (BF₄ ⁻),perfluoroalkylborate, hexafluorophosphate (PF₆ ⁻), andperfluoroalkylphosphate. An example of the monovalent amide anion is(C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 3). An example of a monovalent cyclicamide anion is CF₂(CF₂SO₂)₂N⁻. An example of the monovalent methideanion is (C_(n)F_(2n−1)SO₂)₃C⁻ (n=0 to 3). An example of a monovalentcyclic methide anion is CF₂(CF₂SO₂)₂C⁻(CF₃SO₂). An example ofperfluoroalkylsulfonate anion is (C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4). Anexample of perfluoroalkylborate is {BF_(n)(C_(m)H_(k)F_(2m+1−k))_(4−n)}⁻(n=0 to 3, m=1 to 4, and k=0 to 2m). An example ofperfluoroalkylphosphate is {PF_(n)(C_(m)H_(k)F_(2m+1−k))_(6−n)}⁻ (n=0 to5, m=1 to 4, and k=0 to 2m). Note that the anion is not limited theseexamples.

In the power storage device of one embodiment of the present invention,the ionic liquid is any of the stereoisomers of the ionic liquids shownin General Formulae (G1) to (G3). Isomers are different compounds withthe same molecular formula. Stereoisomers are a particular kind ofisomers in which only the spatial orientation differs but coupling ofatoms is the same. Thus, in this specification and the like, the term“stereoisomers” include enantiomers, geometric (cis-trans) isomers, anddiastereomers which include two or more chiral centers and are notenatiomers.

When a quaternary ammonium cation is used as the organic cation of theionic liquid and bis(fluorosulfonyl)amide (abbreviation: FSA) is used asthe anion thereof, a passivating film can be formed on the surface ofthe negative electrode active material layer, which enables a stablelithium insertion reaction.

When the ionic liquid has low reduction resistance and a carbon materialis used for the negative electrode, the ionic liquid is reduced, whichleads to an increase in initial irreversible capacity. An ionic liquidincluding an aliphatic quaternary ammonium cation has higher reductionresistance than an ionic liquid including an aromatic cation. Therefore,by using the ionic liquid including an aliphatic quaternary ammoniumcation as the ionic liquid contained in the electrolyte solution, a lowpotential negative electrode material such as a highly graphitizedcarbon material can be preferably used.

The electrolyte dissolved in the nonaqueous solvent is a salt at leastincluding ions which serve as carriers and being compatible with thepositive electrode active material layer. As the salt, an alkali metalion or an alkaline earth metal ion can be used. Examples of the alkalimetal ion are a lithium ion, a sodium ion, and a potassium ion. Examplesof the alkaline earth metal ion are a calcium ion, a strontium ion, abarium ion, a beryllium ion, and a magnesium ion. In the case where amaterial containing lithium is used for the positive electrode activematerial layer, a salt containing a lithium ion (hereinafter alsoreferred to as a lithium salt) is preferably selected. In the case wherea material containing sodium is used for the positive electrode activematerial layer, an electrolyte containing sodium is preferably selected.

Examples of the lithium salt are lithium chloride (LiCl), lithiumfluoride (LiF), lithium perchlorate (LiClO₄), fluoroborate (LiBF₄),LiAsF₆, LiPF₆, and Li(CF₃SO₂)₂N.

An ionic liquid including an aliphatic onium cation has higher reductionresistance than an ionic liquid including an aromatic quaternaryammonium cation. Therefore, by using the ionic liquid including thealiphatic onium cation as the ionic liquid contained in the electrolytesolution, a low potential negative electrode material such as a highlygraphitized carbon material can be preferably used.

Further, introduction of a substituent to the aliphatic quaternaryammonium cation decreases the degree of symmetry of the molecule. Thismakes it possible to decrease a melting point of the ionic liquid. Theuse of an electrolyte solution including such an ionic liquid in thepower storage device enables a favorable operation even in a lowtemperature environment.

The use of the ionic liquid including the FSA anion allows a passivatingfilm to be formed on a surface of a negative electrode active materiallayer containing a highly graphitized carbon material, which enables astable lithium insertion reaction in an electrolyte solution containingthe ionic liquid.

The power storage device using the electrolyte solution containing theabove-described ionic liquid preferably has a maximum current density oflower than or equal to 2 mA/g at a voltage within the range of 1.0 V to1.5 V (vs. Li/Li⁺) when cyclic voltammetry measurement is performed at ascan rate of 0.1 mV/sec.

The ionic liquid which can be used for the nonaqueous solvent isdescribed in detail in Embodiment 2.

As the separator, paper; nonwoven fabric; a glass fiber; ceramics; asynthetic fiber containing nylon (polyamide), vinylon (polyvinyl alcoholbased fiber), polyester, acrylic, polyolefin, or polyurethane; or thelike can be used.

FIG. 1B illustrates a power storage device 120.

In the case of stacking the positive electrodes 103 and the negativeelectrodes 106 alternately, to prevent a short circuit between thepositive electrode 103 and the negative electrode 106, bag-likeseparators each holding the positive electrode 103 and the negativeelectrode 106 are sequentially stacked and are put into containers 113and 114 to manufacture the power storage device 120. Alternatively, thepower storage device 120 is manufactured in such a manner that thepositive electrode 103 and the negative electrode 106 each provided witha tab and overlapping with each other with a separator interposedtherebetween are wound and are put into the containers 113 and 114 as awound electrode body.

Next, a coin-type lithium ion secondary battery is described as anotherexample of the power storage device with reference to FIGS. 2A and 2B.FIG. 2A is an external view of the coin-type lithium ion secondarybattery and FIG. 2B is a cross-sectional view thereof.

In a coin-type lithium ion secondary battery 300 in FIG. 2A, a positiveelectrode can 301 doubling as a positive electrode terminal and anegative electrode can 302 doubling as a negative electrode terminal areinsulated from each other and sealed by a gasket 303 made ofpolypropylene or the like. The positive electrode 103 includes thepositive electrode current collector 101 and the positive electrodeactive material layer 102. The negative electrode 106 includes thenegative electrode current collector 104 and the negative electrodeactive material layer 105. A separator 107 and an electrolyte solution(not illustrated) are included between the positive electrode activematerial layer 102 and the negative electrode active material layer 105.

The positive electrode 103, the negative electrode 106, and theseparator 107 in FIG. 2B can have the structures described withreference to FIGS. 1A and 1B.

A corrosion-resistant metal such as nickel, aluminum, or titanium; analloy of such a metal; or an alloy of such a metal and another metal canbe used to form the positive electrode can 301 and the negativeelectrode can 302. Alternatively, the positive electrode can 301 and thenegative electrode can 302 are preferably covered with nickel, aluminum,or the like in order to prevent corrosion caused by the electrolytesolution. The positive electrode can 301 and the negative electrode can302 are electrically connected to the positive electrode 103 and thenegative electrode 106, respectively.

The positive electrode 103, the negative electrode 106, and theseparator 107 are impregnated with the electrolyte solution. Then, asillustrated in FIG. 2B, the positive electrode can 301, the positiveelectrode 103, the separator 107, the negative electrode 106, and thenegative electrode can 302 are stacked in this order with the positiveelectrode can 301 positioned at the bottom. The positive electrode can301 and the negative electrode can 302 are subjected to pressure bondingwith the gasket 303 provided therebetween. Thus, the coin-type lithiumion secondary battery is manufactured.

In the power storage device of one embodiment of the present invention,the content percentage of the less-graphitized carbon material in thenegative electrode active material layer 105 is decreased (the contentpercentage is less than 2 wt %, preferably zero). Thus, the cationspecies of the ionic liquid can be inhibited from being inserted intothe less-graphitized carbon material. In addition, the ionic liquid canbe inhibited from reacting with the surface of the negative electrodeactive material to be decomposed. This makes it possible to suppress anincrease in the initial irreversible capacity of the power storagedevice.

This embodiment can be implemented combining with any of the otherembodiments as appropriate.

Embodiment 2

In this embodiment, an ionic liquid which can be used in an electrolytesolution of the power storage device of one embodiment of the presentinvention is described in detail.

The ionic liquid which can be used in the electrolyte solution iscomposed of an organic cation and an anion. As the ionic liquid, thefollowing can be used.

As the ionic liquid, an ionic liquid composed of a quaternary ammoniumcation and a monovalent anion and represented by General Formula (G1)can be used, for example.

In General Formula (G1), R¹ to R⁶ separately represent an alkyl grouphaving 1 to 20 carbon atoms, a methoxy group, a methoxymethyl group, amethoxyethyl group, or a hydrogen atom.

As the ionic liquid, an ionic liquid composed of a quaternary ammoniumcation and a monovalent anion and represented by General Formula (G2)can be used, for example.

In General Formula (G2), R⁷ to R¹³ separately represent an alkyl grouphaving 1 to 20 carbon atoms, a methoxy group, a methoxymethyl group, amethoxyethyl group, or a hydrogen atom.

As the ionic liquid, an ionic liquid composed of a quaternary ammoniumcation and a monovalent anion and represented by General Formula (G3)can be used, for example.

In General Formula (G3), each of n and m is greater than or equal to 1and less than or equal to 3, and each of α and β is greater than orequal to 0 and less than or equal to 6. When n is 1, α is greater thanor equal to 0 and less than or equal to 4. When n is 2, α is greaterthan or equal to 0 and less than or equal to 5. When n is 3, α isgreater than or equal to 0 and less than or equal to 6. When in is 1, βis greater than or equal to 0 and less than or equal to 4. When m is 2,β is greater than or equal to 0 and less than or equal to 5. When m is3, β is greater than or equal to 0 and less than or equal to 6. Notethat “α or β is 0” means that at least one of two aliphatic rings isunsubstituted. Note that the case where both α and β are 0 is excluded.X or Y is a substituent such as a straight chain or lateral chain alkylgroup having 1 to 4 carbon atoms, a straight chain or lateral chainalkoxy group having 1 to 4 carbon atoms, or a straight chain or lateralchain alkoxyalkyl group having 1 to 4 carbon atoms. Further, A⁻represents a monovalent amide anion, a monovalent methide anion, aperfluoroalkylsulfonate anion, tetrafluoroborate, perfluoroalkylborate,hexafluorophosphate, or perfluoroalkylphosphate.

In a quaternary Spiro ammonium cation, two aliphatic rings that composea Spiro ring are each a five-membered ring, a six-membered ring, or aseven-membered ring.

As an example of the quaternary ammonium cation represented by GeneralFormula (G3), a quaternary ammonium cation -having a Spiro ringincluding a five-membered ring can be given. An ionic liquid includingsuch a quaternary ammonium cation is represented by General Formula(G4).

In General Formula (G4), R¹⁴ to R²¹ separately represent a hydrogenatom, a straight chain or lateral chain alkyl group having 1 to 4 carbonatoms, a straight chain or lateral chain all oxy group having 1 to 4carbon atoms, or a straight chain or lateral chain alkoxyalkyl grouphaving 1 to 4 carbon atoms.

As the ionic liquid, an ionic liquid represented by General Formula (G5)shown below can be used, for example.

In General Formula (G5), R²² to R³⁰ separately represent a hydrogenatom, a straight chain or lateral chain alkyl group having 1 to 4 carbonatoms, a straight chain or lateral chain alkoxy group having 1 to 4carbon atoms, or a straight chain or lateral chain alkoxyalkyl grouphaving 1 to 4 carbon atoms.

As the ionic liquid., an ionic liquid represented by General Formula(G6) shown below can be used, for example.

In General Formula (G6), R³¹ to R⁴⁰ separately represent a hydrogenatom, a straight chain or lateral chain alkyl group having 1 to 4 carbonatoms, a straight chain or lateral chain alkoxy group having 1 to 4carbon atoms, or a straight chain or lateral chain alkoxyalkyl grouphaving 1 to 4 carbon atoms.

As the ionic liquid, an ionic liquid represented by General Formula (G7)shown below can be used, for example.

In General Formula (G7), R⁴¹ to R⁵⁰ separately represent a hydrogenatom, a straight chain or lateral chain alkyl group having 1 to 4 carbonatoms, a straight chain or lateral chain alkoxy group having 1 to 4carbon atoms, or a straight chain or lateral chain alkoxyalkyl grouphaving 1 to 4 carbon atoms.

As the ionic liquid, an ionic liquid represented by General Formula (G8)shown below can be used, for example.

In General Formula (G8), R⁵¹ to R⁶¹ separately represent a hydrogenatom, a straight chain or lateral chain alkyl group having 1 to 4 carbonatoms, a straight chain or lateral chain alkoxy group having 1 to 4carbon atoms, or a straight chain or lateral chain alkoxyalkyl grouphaving 1 to 4 carbon atoms.

As the ionic liquid, an ionic liquid represented by General Formula (G9)shown below can be used, for example.

In General Formula (G9), R⁶² to R⁷³ separately represent a hydrogenatom, a straight chain or lateral chain alkyl group having 1 to 4 carbonatoms, a straight chain or lateral chain alkoxy group having 1 to 4carbon atoms, or a straight chain or lateral chain alkoxyalkyl grouphaving 1 to 4 carbon atoms.

Examples of an anion in each of General Formulae (G1) to (G9) are amonovalent amide anion, a monovalent methide anion, a fluorosulfonateanion (SO₃F⁻), a perfluoroalkylsulfonate anion, tetrafluoroborate (BF₄⁻), perfluoroalkylborate, hexafluorophosphate (PF₆ ⁻), andperfluoroalkylphosphate. An example of the monovalent amide anion is(C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 3). An example of a monovalent cyclicamide anion is CF₂(CF₂SO₂)₂N⁻. An example of the monovalent methideanion is (C_(n)F_(2n+1)So₂)₃C⁻ (n=0 to 3). An example of a monovalentcyclic methide anion is CF₂(CF₂SO₂)₂C⁻(CF₃SO₂). An example ofperfluoroalkylsulfonate anion is (C_(m)F_(2m+1)SO₃)⁻ (m=0 to 4), Anexample of perfluoroalkylborate is {BF_(n)(C_(m)H_(k)F_(2m+1−k))_(4−n)}⁻(n=0 to 3, m=1 to 4, and k=0 to 2m). An example ofperfluoroalkylphosphate is {PF_(n)(C_(m)H_(k)F_(2m+1−k))_(6−n)}⁻ (n=0 to5, m=1 to 4, and k=0 to 2m). Note that the anion is not limited theseexamples.

Specific examples of the ionic liquid are organic compounds representedby Structural Formulae (101) to (120), Structural Formulae (201) to(230), Structural Formulae (301) to (327), Structural Formulae (401) to(457), Structural Formulae (501) to (605), and Structural Formulae (701)to (709).

Pyrrolidinium ionic liquids are represented by Structural Formulae (101)to (120).

Piperidinium ionic liquids are represented by Structural Formulae (201)to (230).

Spiro quaternary ammonium ionic liquids are represented by Structural.Formulae (301) to (327), Structural Formulae (401) to (457), StructuralFormulae (501) to (605), and Structural Formulae (701) to (709).

Further, examples of an anion in each of in Structural Formulae (101) to(120), Structural Formulae (201) to (230), Structural Formulae (301) to(327), Structural Formulae (401) to (457), Structural Formulae (501) to(605), and Structural Formulae (701) to (709) are a monovalent amideanion, a monovalent methide anion, a fluorosulfonate anion (SO₃F⁻), aperfluoroalkylsulfonate anion, tetrafluoroborate (BF₄ ⁻),perfluoroalkylborate, hexafluorophosphate (PF₆ ⁻), andperfluoroalkylphosphate. An example of the monovalent amide anion is(C_(n)F_(2n+1)SO₂)₂N⁻ (n=0 to 3). An example of a monovalent cyclicamide anion is CF₂(CF₂SO₂)₂N⁻. An example of the monovalent methideanion is (CF₂(CF₂SO₂)₃C⁻ (n=0 to 3). An example of a monovalent cyclicmethide anion is CF₂(CF₂SO₂)₂C⁻(CF₃SO₂). An example ofperfluoroalkylsulfonate anion is (C_(m)F_(2m+1)SO₃)⁻ (n=0 to 4). Anexample of perfluoroalkylborate is {BF_(n)(C_(m)H_(k)F_(2m+1−k))_(4−n)}⁻(n=0 to 3, m=1 to 4, and k=0 to 2m). An example ofperfluoroalkylphosphate is {PF_(n)(C_(m)H_(k)F_(2m+1−k))_(6−n)}⁻ (n=0 to5, m=1 to 4, and k=0 to 2m). Note that the anion is not limited theseexamples.

In the power storage device of one embodiment of the present invention,the ionic liquid is any of the stereoisomers of the ionic liquids shownin Structural Formulae (101) to (120), Structural Formulae (201) to(230), Structural Formulae (301) to (327), Structural Formulae (401) to(457), Structural. Formulae (501) to (605), and Structural Formulae(701) to (709). Isomers are different compounds with the same molecularformula. Stereoisomers are a particular kind of isomers in which onlythe spatial orientation differs but coupling of atoms is the same. Thus,in this specification and the like, the term “stereoisomers” includeenantiomers, geometric (cis-trans) isomers, and diastereomers whichinclude two or more chiral centers and are not enatiomers.

When a quaternary ammonium cation is used as the organic cation of theionic liquid and bis(fluorosulfonyl)amide (abbreviation: FSA) is used asthe anion thereof, a passivating film can be formed on the surface ofthe negative electrode active material layer, which enables a stablelithium insertion reaction.

When the ionic liquid has low reduction resistance and a carbon materialis used for the negative electrode, the ionic liquid is reduced, whichleads to an increase in initial irreversible capacity. An ionic liquidincluding an aliphatic quaternary ammonium cation has higher reductionresistance than an ionic liquid including an aromatic cation. Therefore,by using the ionic liquid including an aliphatic quaternary ammoniumcation as the ionic liquid contained in the electrolyte solution, a lowpotential negative electrode material such as a highly graphitizedcarbon material can be preferably used.

Further, as shown in Structural Formulae (101) to (120), StructuralFormulae (201) to (230), Structural Formulae (301) to (327), StructuralFormulae (401) to (457), and Structural Formulae (501) to (605),introduction of a substituent to the quaternary ammonium cationdecreases the degree of symmetry of the molecule. Accordingly, themelting point of the ionic liquid tends to decrease. For example,introduction of a methyl group to a pyrrolidine skeleton decreases themelting point to −10° C. or lower, preferably −30° C. or lower. At atemperature lower than the melting point of the ionic liquid, anincrease in resistance due to solidification of the ionic liquid can besuppressed. The use of an electrolyte solution including such an ionicliquid enables a power storage device to operate favorably even in a lowtemperature environment.

The power storage device using the electrolyte solution containing theabove-described ionic liquid preferably has a maximum current density oflower than or equal to 2 mA/g at a voltage within the range of 1.0 V to1.5 V (vs., Li/Li⁺) when cyclic voltammetry measurement is performed ata scan rate of 0.1 mV/sec.

This embodiment can be implemented combining with any of the otherembodiments as appropriate.

Embodiment 3

The power storage device of one embodiment of the present invention canbe used as a power source for a variety of electric appliances which canoperate with electric power.

Specific examples of electric appliances each using the power storagedevice of one embodiment of the present invention are as follows:display devices of televisions, monitors, and the like, lightingdevices, desktop personal computers and laptop personal computers, wordprocessors, image reproduction devices which reproduce still images andmoving images stored in recording media such as digital versatile discs(DVDs), portable CD players, portable radios, tape recorders, headphonestereos, stereos, table clocks, wall clocks, cordless phone handsets,transceivers, mobile phones, car phones, portable game machines,calculators, portable information terminals, electronic notebooks,e-book readers, electronic translators, audio input devices, videocameras, digital still cameras, electric shavers, high-frequency heatingappliances such as microwave ovens, electric rice cookers, electricwashing machines, electric vacuum cleaners, water heaters, electric EMS,hair dryers, air-conditioning systems such as air conditioners,humidifiers, and dehumidifiers, dishwashers, dish dryers, clothesdryers, futon dryers, electric refrigerators, electric freezers,electric refrigerator-freezers, freezers for preserving DNA,flashlights, electric power tools such as chain saws, smoke detectors,and medical equipment such as dialyzers. The examples also includeindustrial equipment such as guide lights, traffic lights, beltconveyors, elevators, escalators, industrial robots, power storagesystems, and power storage devices for leveling the amount of electricpower supply and smart grid. In addition, moving objects driven by anelectric motor using electric power from a power storage device are alsoincluded in the category of electric appliances. Examples of the movingobjects are electric vehicles (EV), hybrid electric vehicles (HEV) whichinclude both an internal-combustion engine and a motor, plug-in hybridelectric vehicles (PHEV), tracked vehicles in which caterpillar tracksare substituted for wheels of these vehicles, motorized bicyclesincluding motor-assisted bicycles, motorcycles, electric wheelchairs,golf carts, boats, ships, submarines, helicopters, aircraft, rockets,artificial satellites, space probes, planetary probes, and spacecrafts.

In the above electric appliances, the power storage device of oneembodiment of the present invention can be used as a main power sourcefor supplying enough electric power for almost the whole powerconsumption. Alternatively, in the above electric appliances, the powerstorage device of one embodiment of the present invention can be used asan uninterruptible power source which can supply electric power to theelectric appliances when the supply of electric power from the mainpower source or a commercial power source is stopped. Stillalternatively, in the above electric appliances, the power storagedevice of one embodiment of the present invention can be used as anauxiliary power source for supplying electric power to the electricappliances at the same time as the electric power supply from the mainpower source or a commercial power source.

FIG. 3 illustrates specific structures of the electric appliances. InFIG. 3, a display device 8000 is an example of an electric applianceusing a power storage device 8004 of one embodiment of the presentinvention. Specifically, the display device 8000 corresponds to adisplay device for TV broadcast reception and includes a housing 8001, adisplay portion 8002, speaker portions 8003, the power storage device8004, and the like. The power storage device 8004 of one embodiment ofthe present invention is provided in the housing 8001. The displaydevice 8000 can receive electric power from a commercial power source.Alternatively, the display device 8000 can use electric power stored inthe power storage device 8004. Thus, the display device 8000 can beoperated with the use of the power storage device 8004 of one embodimentof the present invention as an uninterruptible power source even whenelectric power cannot be supplied from a commercial power source due topower failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like in addition to TV broadcast reception.

In FIG. 3, an installation lighting device 8100 is an example of anelectric appliance including a power storage device 8103 of oneembodiment of the present invention. Specifically, the lighting device8100 includes a housing 8101, a light source 8102, the power storagedevice 8103, and the like. Although FIG. 3 illustrates the case wherethe power storage device 8103 is provided in a ceiling 8104 on which thehousing 8101 and the light source 8102 are installed, the power storagedevice 8103 may be provided in the housing 8101. The lighting device8100 can receive electric power from a commercial power source.Alternatively, the lighting device 8100 can use electric power stored inthe power storage device 8103. Thus, the lighting device 8100 can beoperated with the use of the power storage device 8103 of one embodimentof the present invention as an uninterruptible power source even whenelectric power cannot be supplied from a commercial power source due topower failure or the like.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 3 as an example, the power storagedevice of one embodiment of the present invention can be used in aninstallation lighting device provided in, for example, a wall 8105, afloor 8106, a window 8107, or the like other than the ceiling 8104.Alternatively, the power storage device can be used in a tabletoplighting device or the like.

As the light source 8102, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, and alight-emitting, element such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 3, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electric appliance using a powerstorage device 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, the power storage device 8203, and the like. Although FIG.3 illustrates the case where the power storage device 8203 is providedin the indoor unit 8200, the power storage device 8203 may be providedin the outdoor unit 8204. Alternatively, the power storage devices 8203may be provided in both the indoor unit 8200 and the outdoor unit 8204.The air conditioner can receive electric power from the commercial powersource. Alternatively, the air conditioner can use electric power storedin the power storage device 8203. Particularly in the case where thepower storage devices 8203 are provided in both the indoor unit 8200 andthe outdoor unit 8204, the air conditioner can be operated with the useof the power storage device 8203 of one embodiment of the presentinvention as an uninterruptible power source even when electric powercannot be supplied from a commercial power source due to power failureor the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 3 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 3, an electric refrigerator-freezer 8300 is an example of anelectric appliance using a power storage device 8304 of one embodimentof the present invention. Specifically, the electricrefrigerator-freezer 8300 includes a housing 8301, a door for arefrigerator 8302, a door for a freezer 8303, the power storage device8304, and the like. The power storage device 8304 is provided inside thehousing 8301 in FIG. 3. Alternatively, the electric refrigerator-freezer8300 can use electric power stored in the power storage device 8304.Thus, the electric refrigerator-freezer 8300 can be operated with theuse of the power storage device 8304 of one embodiment of the presentinvention as an uninterruptible power source even when electric powercannot be supplied from a commercial power source due to power failureor the like.

Note that among the electric appliances described above, thehigh-frequency heating appliances such as microwave ovens and theelectric rice cookers require high electric power in a short time. Thetripping of a circuit breaker of a commercial power source in use ofelectric appliances can be prevented by using the power storage deviceof one embodiment of the present invention as an auxiliary power sourcefor supplying electric power which cannot be supplied enough by acommercial power source.

In addition, in a time period when electric appliances are not used,specifically when the proportion of the amount of electric power whichis actually used to the total amount of electric power which can besupplied by a commercial power source (such a proportion referred to asusage rate of electric power) is low, electric power can be stored inthe power storage device, whereby the usage rate of electric power canbe reduced in a time period when the electric appliances are used. Forexample, in the case of the electric refrigerator-freezer 8300, electricpower can be stored in the power storage device 8304 in night time whenthe temperature is low and the door for a refrigerator 8302 and the doorfor a freezer 8303 are not often opened or closed. On the other hand, indaytime when the temperature is high and the door for a refrigerator8302 and the door for a freezer 8303 are frequently opened and closed,the power storage device 8304 is used as an auxiliary power source;thus, the usage rate of electric power in daytime can be reduced.

This embodiment can be implemented combining with any of the otherembodiments as appropriate.

Embodiment 4

Next, a portable information terminal which is an example of an electricappliance is described with reference to FIGS. 4A to 4C.

FIGS. 4A and 4B illustrate a foldable tablet terminal. In FIG. 4A, thetablet terminal is open (unfolded) and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a switch 9034 forswitching display modes, a power switch 9035, a switch 9036 forswitching to power-saving mode, a fastener 9033, and an operation switch9038.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Note that FIG. 4A shows, as an example, that half of the areaof the display portion 9631 a has only a display function and the otherhalf of the area has a touch panel function. However, the structure ofthe display portion 9631 a is not limited to this, and all the area ofthe display portion 9631 a may have a touch panel function. For example,all the area of the display portion 9631 a can display keyboard buttonsand serve as a touch panel while the display portion 9631 b can be usedas a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a finger, a stylus, or the liketouches the place where a button 9639 for switching to keyboard displayis displayed in the touch panel, keyboard buttons can be displayed onthe display portion 9631 b.

Touch input can be performed on the touch panel regions 9632 a and 9632b at the same time.

The switch 9034 for switching display modes can switch displayorientation (e.g., between landscape mode and portrait mode) and selecta display mode (switch between monochrome display and color display),for example. With the switch 9036 for switching to power-saving mode,the luminance of display can be optimized depending on the amount ofexternal light at the time when the tablet terminal is in use, which issensed with an optical sensor incorporated in the tablet terminal. Thetablet terminal may include another detection device such as a sensorfor sensing orientation (e.g., a gyroscope or an acceleration sensor) inaddition to the optical sensor.

Note that FIG. 4A illustrates an example in which the display portion9631 a and the display portion 9631 b have the same display area;however, without limitation thereon, one of the display portions may bedifferent from the other display portion in size and display quality.For example, one of them may be a display panel that can displayhigher-definition images than the other.

In FIG. 4B, the tablet terminal is close (folded) and includes thehousing 9630, a solar cell 9633, a charge and discharge control circuit9634, a battery 9635, and a DCDC converter 9636. Note that FIG. 4Billustrates an example in which the charge and discharge control circuit9634 includes the battery 9635 and the DCDC converter 9636. The battery9635 includes the power storage device described in the aboveembodiment.

Since the tablet terminal can be folded in two, the housing 9630 can beclosed when the tablet terminal is not in use. Thus, the displayportions 9631 a and 9631 b can be protected, thereby providing thetablet terminal with excellent endurance and excellent reliability forlong-term use.

The tablet terminal illustrated in FIGS. 4A and 4B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 9633is preferably provided on one or both surfaces of the housing 9630, inwhich case the battery 9635 can be charged efficiently. The use of thepower storage device of one embodiment of the present invention as thebattery 9635 brings an advantage such as a reduction in size.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 4B are described with reference to a blockdiagram of FIG. 4C. The solar cell 9633, the battery 9635, the DCDCconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are illustrated in FIG. 4C, and the battery 9635, the DCDCconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 4B.

First, an example of the operation in the case where electric power isgenerated by the solar cell 9633 using external light is described. Thevoltage of electric power generated by the solar cell is raised orlowered by the DCDC converter 9636 so that the electric power has avoltage for charging the battery 9635. Then, when the electric powerfrom the solar cell 9633 is used for the operation of the displayportion 9631, the switch SW1 is turned on and the voltage of theelectric power is raised or lowered by the converter 9637 so as to be avoltage needed for the display portion 9631. In addition, when displayon the display portion 9631 is not performed, the switch SW1 is turnedoff and the switch SW2 is turned on so that charge of the battery 9635may be performed.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, without limitation on the power generationmeans, the battery 9635 may be charged using another power generationmeans such as a piezoelectric element or a thermoelectric conversionelement (Peltier element). For example, the battery 9635 may be chargedwith a non-contact power transmission module that transmits and receivespower wirelessly (without contact) to charge the battery or with acombination of other charging means.

It is needless to say that one embodiment of the present invention isnot limited to the electric appliance illustrated in FIGS. 4A to 4C aslong as the electric appliance is equipped with the power storage devicedescribed in the above embodiment.

This embodiment can be implemented combining with any of the otherembodiments as appropriate.

Embodiment 5

An example of the moving object which is an example of the electricappliances is described with reference to FIGS. 5A and 5B.

The power storage device described in any of the above embodiments canbe used as a control battery. The control battery can be externallycharged by electric power supply using a plug-in technique orcontactless power feeding. Note that in the case where the moving objectis an electric railway vehicle, the electric railway vehicle can becharged by electric power supply from an overhead cable or a conductorrail.

FIGS. 5A and 5B illustrate an example of an electric vehicle. Anelectric vehicle 9700 is equipped with a power storage device 9701. Theoutput of electric power of the power storage device 9701 is controlledby a control circuit 9702 and the electric power is supplied to adriving device 9703. The control circuit 9702 is controlled by aprocessing unit 9704 including a ROM, a RAM, a CPU, or the like which isnot illustrated.

The driving device 9703 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 9704 outputs a control signal to the control circuit 9702 based oninput data such as data on operation (e.g., acceleration, deceleration,or stop) by a driver of the electric vehicle 9700 or data on driving theelectric vehicle 9700 (e.g., data on an upgrade or a downgrade, or dataon a load on a driving wheel). The control circuit 9702 adjusts theelectric energy supplied from the power storage device 9701 inaccordance with the control signal of the processing unit 9704 tocontrol the output of the driving device 9703. In the case where the ACmotor is mounted, although not illustrated, an inverter which convertsdirect current into alternate current is also incorporated.

The power storage device 9701 can be charged by external electric powersupply using a plug-in technique. For example, the power storage device9701 is charged by a commercial power source through a power plug. Thepower storage device 9701 can be charged by converting external powerinto DC constant voltage having a predetermined voltage level through aconverter such as an AC-DC converter. Providing the power storage deviceof one embodiment of the present invention as the power storage device9701 can contribute to an increase in the capacity of the battery, sothat convenience can be improved. Moreover, the higher charging anddischarging rate of the power storage device 9701 can contribute togreater acceleration and excellent performance of the electric vehicle9700. When the power storage device 9701 itself can be more compact andmore lightweight as a result of improved characteristics of the powerstorage device 9701, the vehicle can be lightweight, leading to anincrease in fuel efficiency.

This embodiment can be implemented combining with any of the otherembodiments as appropriate.

EXAMPLE 1

In this example, description is given of the comparison results ofcharge and discharge characteristics in a lithium ion secondary batteryincluding a negative electrode active material layer which does notcontain a less-graphitized carbon material and a lithium ion secondarybattery including a negative electrode active material layer containingthe less-graphitized carbon material.

First, coin-type lithium ion secondary batteries manufactured in thisexample are described with reference to FIGS. 2A and 2B.

First, with the use of a positive electrode active material, aconductive additive, a binder, and a disperse medium, a positiveelectrode paste was formed. The positive electrode paste was applied onto the positive electrode current collector 101 and dried. Thus, thepositive electrode 103 including the positive electrode active materiallayer 102 was formed.

In this example, lithium iron phosphate (LiFePO₄) was used as thepositive electrode active material, acetylene black was used as theconductive additive, and polyvinylidene fluoride (PVDF) was used as thebinder. Lithium iron phosphate, acetylene black, and polyvinylidenefluoride were mixed in a ratio of 85:8:7. As the disperse medium forviscosity adjustment, NMP was added to and mixed with the mixture. Thus,the positive electrode paste was formed.

The positive electrode paste formed by the above method was applied tothe positive electrode current collector 101 (20-μm-thick aluminum) anddried at 135° C. in a reduced pressure environment for 2 hours, and thendried at 170° C. in a reduced pressure environment for 10 hours, wherebythe positive electrode active material layer 102 was formed. Note thatin the positive electrode 103, the amount of lithium iron phosphate was6.4 mg/cm², and the single-electrode theoretical capacity was 1.1mAh/cm². The positive electrode active material layer 102 had athickness of approximately 50 μm. Then, the positive electrode 103 wasstamped out into a round shape.

Next, with the use of a negative electrode active material, a binder,and a disperse medium, a negative electrode paste was formed. Thenegative electrode paste was applied on to the negative electrodecurrent collector 104 and dried. Thus, the negative electrode 106including the negative electrode active material layer 105 was formed.

In this example, mesocarbon microbeads with a particle diameter of 9 μmwere used as the negative electrode active material, and PVDF was usedas the binder. Mesocarbon microbeads and PVDF were mixed in a ratio of90:10. As the disperse medium for viscosity adjustment, NMP was added toand mixed with the mixture. Thus, the negative electrode paste wasformed.

The negative electrode paste formed by the above method was applied tothe negative electrode current collector 104 (18-μm-thick copper) anddried at 70° C. in an air atmosphere for 30 minutes, and then dried at170° C. in a reduced pressure environment for 10 hours, whereby thenegative electrode active material layer 105 was formed. Note that inthe negative electrode 106, the amount of mesocarbon microbeads was 8.1mg/cm⁻¹, and the single-electrode theoretical capacity was 3.0 mAh/cm².The negative electrode active material layer 105 had a thickness ofapproximately 85 μm. Then, the negative electrode 106 was stamped outinto a round shape.

In an electrolyte solution, N-methyl-N-propylpyrrolidiniumbis(fluorosulfonyl)amide (abbreviation: P13-FSA) was used as anonaqueous solvent and lithium bis(trifluoromethanesulfonyl)amide(hereinafter abbreviated to LiTFSA) was used as an electrolyte. Asolution in which 1M LiTFSA was dissolved in P13-FSA was used.

As the separator 107, a polyvinylidene fluoride film (Durapore filter)with a thickness of 125 μm subjected to hydrophilic treatment was used.The separator was impregnated with the electrolyte solution to be used.

The positive electrode can 301 and the negative electrode can 302 wereformed of stainless steel (SUS). As the gasket 303, a spacer or a washerwas used.

As illustrated in FIGS. 2A and 2B, the positive electrode can 301, thepositive electrode 103, the separator 107, the negative electrode 106,the gasket 303, and the negative electrode can 302 were stacked, and thepositive electrode can 301 and the negative electrode can 302 werecrimped to each other with a “coin cell crimper”.

Thus, the coin-type lithium ion secondary battery was manufactured. Themanufactured coin-type lithium ion secondary battery is Sample A.

Next, Sample B, Sample C, Sample D, Sample E, and Sample F eachincluding a nonaqueous solvent for an electrolyte solution differentfrom that in Sample A were manufactured. The nonaqueous solvent for theelectrolyte solution in Sample B was N-methyl-N-n-propylpiperidiniumbis(fluorosulfonyl)amide (abbreviation: PP13-FSA); the nonaqueoussolvent for the electrolyte solution in Sample C,1-ethyl-1,3-dimethylpyrrolidinium bis(fluorosulfonyl)amide(abbreviation: 3mP12-FSA); the nonaqueous solvent for the electrolytesolution in Sample D, 1,3-dimethyl-1-n-propylpiperidiniumbis(fluorosulfonyl)amide (abbreviation: 3mPP13-FSA); the nonaqueoussolvent for the electrolyte solution in Sample E, 2mAS44-FSA; and thenonaqueous solvent for the electrolyte solution in Sample F, EC/DEC.Other than the structure of the nonaqueous solvent for the electrolytesolution, Sample B, Sample C, Sample D, Sample E, and Sample F each hada structure similar to that of Sample A.

Next, as comparative examples, lithium ion secondary batteries eachincluding a negative electrode active material layer in which acetyleneblack was contained as a conductive additive were manufactured.

In a negative electrode paste used for the comparative examples,mesocarbon microbeads with a particle diameter of 9 μm were used as anegative electrode active material, and PVDF was used as a binder. Inaddition, acetylene black (AB) was used as the conductiveadditive-Mesocarbon microbeads, AB, and PVDF were mixed in a ratio of93:2:5. As a disperse medium for viscosity adjustment, NMP was added toand mixed with the mixture. Thus, the negative electrode paste wasformed.

The negative electrode paste formed by the above method was applied tothe negative electrode current collector 104 (18-μm-thick copper) anddried at 70° C. in an air atmosphere for 30 minutes, and then dried at170° C. in a reduced pressure environment for 10 hours, whereby thenegative electrode active material layer 105 was formed. Note that inthe negative electrode 106, the amount of the mesocarbon microbeads was9.4 mg/cm², and the single-electrode theoretical capacity was 3.5mAh/cm². The negative electrode active material layer 105 had athickness of approximately 85 μm. Then, the negative electrode 106 wasstamped out into a round shape.

The positive electrode 103 was formed in a manner similar to that ofSample A. Note that the amount of lithium iron phosphate in the positiveelectrode 103 was approximately 6.4 mg/cm² and the single-electrodetheoretical capacity was 1.1 mAh/cm². The positive electrode activematerial layer 102 had a thickness of approximately 50 μm. Then, thepositive electrode 103 was stamped out into a round shape.

Next, as in the case of Sample A, in an electrolyte solution, P13-FSAwas used as a nonaqueous solvent and LiTFSA was used as an electrolyte.A solution in which 1M LiTFSA was dissolved in P13-FSA was used.

As the separator 107, a polyvinylidene fluoride film (Durapore filter)with a thickness of 125 μm subjected to hydrophilic treatment was used.Note that the separator 107 was impregnated with the electrolytesolution.

The positive electrode can 301 and the negative electrode can 302 wereformed of stainless steel (SUS). As the gasket 303, a spacer or a washerwas used.

As illustrated in FIGS. 2A and 2B, the positive electrode can 301, thepositive electrode 103, the separator 107, the negative electrode 106,the gasket 303, and the negative electrode can 302 were stacked, and thepositive electrode can 301 and the negative electrode can 302 werecrimped to each other with a “coin cell crimper”. Thus, the coin-typelithium ion secondary battery was manufactured. The manufacturedcoin-type lithium ion secondary battery is Comparative Sample G.

Next, Comparative Sample H, Comparative Sample I, Comparative Sample J,Comparative Sample K, and Comparative Sample each including a nonaqueoussolvent for an electrolyte solution different from that of ComparativeSample G were manufactured. The nonaqueous solvent for the electrolytesolution in Comparative Sample H was PP13-FSA; the nonaqueous solventfor the electrolyte solution in Comparative Sample I, 3mP12-FSA; thenonaqueous solvent for the electrolyte solution in Comparative Sample J,3mP13-FSA; the nonaqueous solvent for the electrolyte solution inComparative Sample K, 2mAS44-FSA; and the nonaqueous solvent for theelectrolyte solution in Comparative Sample EC/DEC. Other than thestructure of the nonaqueous solution for the electrolyte solution,Comparative Sample H, Comparative Sample I, Comparative Sample J,Comparative Sample K, and Comparative Sample L each had a structuresimilar to that of Comparative Sample G.

Here, Structural Formulae of the ionic liquids used in Samples A to Fand Comparative Samples G to L are shown below.

Next, Samples A to F and Comparative Samples G to L were each subjectedto a charge and discharge test. Each charge and discharge test wasperformed at 60° C. with a battery charge/discharge tester HJ-1010D8produced by HOKUTO DENKO CORPORATION. Through charge and discharge at acurrent of 0.1 mA and voltages ranging from 2.0 V to 40 V, capacity(mAh) at a rate of 0.1 C was measured. FIGS. 6A, 6B, 7A, 7B, 8A, and 8Bshow results of the charge and discharge tests.

FIG. 6A shows the results of Sample A and Comparative Sample G eachusing P13-FSA as the nonaqueous solvent for the electrolyte solution. InFIG. 6A, the thick lines show the results of Sample A; the thin lines,the results of Comparative Sample G FIG. 61B shows the results of SampleB and Comparative Sample H each using 3mPP13-FSA as the nonaqueoussolvent for the electrolyte solution. In FIG. 6B, the thick lines showthe results of Sample B; the thin lines, the results of ComparativeSample H.

FIG. 7A shows the results of Sample C and Comparative Sample I eachusing PP13-FSA as the nonaqueous solvent for the electrolyte solution.In FIG. 7A, the thick lines show the results of Sample C; the thinlines, the results of Comparative Sample I. FIG. 7B shows the results ofSample D and Comparative Sample J each using 3mP12-FSA as the nonaqueoussolvent for the electrolyte solution. In FIG. 7B, the thick lines showthe results of Sample D; the thin lines, the results of ComparativeSample J.

FIG. 8A shows the results of Sample E and Comparative Sample K eachusing 2mAS44-FSA as the nonaqueous solvent for the electrolyte solution.In FIG. 8A, the thick lines show the results of Sample E; the thinlines, the results of Comparative Sample K. FIG. 8B shows the results ofSample F and Comparative Sample L each using EC/DEC as the nonaqueoussolvent for the electrolyte solution. In FIG. 8B, the thick lines showthe results of Sample F; the thin lines, the results of ComparativeSample L.

The results in FIGS. 6A, 6B, 7A, 7B, 8A, and 8B show that among thesecondary batteries using the ionic liquid as the nonaqueous solvent forthe electrolyte solution, a decrease in the capacity of each ofComparative Samples G to K was more significant than that in thecapacity of each of Samples A to E. Further, in the secondary batteriesusing EC/DEC as the nonaqueous solvent for the electrolyte solution,Comparative Sample L had lower capacity than Sample F.

Here, description is given of results of Raman spectra of acetyleneblack and mesocarbon microbeads measured by Kanas spectrometry. A PLmicroscope (LabRAM HR-PL) manufactured by HORIBA, Ltd. was used for themeasurement.

FIG. 10 shows the Raman spectrum of acetylene black. The vertical axisrepresents intensity and the horizontal axis represents Raman shift. Asshown in FIG. 10, a D band with a peak around 1360 cm⁻¹ and a G bandwith a peak around 1580 cm⁻¹ were observed. Further, when the intensityof the peak around 1360 cm⁻¹ was represented by I₁₃₆₀ and the intensityof the peak around 1580 cm⁻¹ was represented by I₁₅₈₀, an R value thatrefers to the ratio of the peak intensity I₁₃₆₀ to the peak intensityI₁₅₈₀ (I₁₃₆₀/I₁₅₈₀) was 1.2.

FIG. 11 shows the Raman spectrum of mesocarbon microbeads. The verticalaxis represents intensity and the horizontal axis represents Ramanshift. As shown in FIG. 11., a D band with a peak around 1360 cm⁻¹ and aU band with a peak around 1580 cm⁻¹ were observed. Further, when theintensity of the peak around 1360 cm⁻¹ was represented by I₁₃₆₀ and theintensity of the peak around 1580 cm⁻¹ was represented by I₁₅₈₀, an Rvalue that refers to the ratio of the peak intensity I₁₃₆₀ to the peakintensity I₁₅₈₀ (I₁₃₆₀/I₁₅₈₀) was 0.2.

The results in FIGS. 10 and 11 show that acetylene black is a carbonmaterial which is less graphitized than mesocarbon microbeads. Further,the results in FIGS. 6A, 6B, 7A, 7B, 8A, 8B, and 10 indicate that in thecase of a lithium ion secondary battery including an ionic liquid as anonaqueous solvent for an electrolyte solution, a degree ofgraphitization of a carbon material contained in a negative electrodeactive material layer had great influence on initial irreversiblecapacity.

The above results suggest that in a secondary battery using an ionicliquid as a nonaqueous solvent for an electrolyte solution, the use of anegative electrode active material layer which does not contain aless-graphitized carbon material suppresses an increase in initialirreversible capacity.

EXAMPLE 2

In this example, description is given of results of measuring secondarybatteries using an ionic liquid as a nonaqueous solvent for anelectrolyte solution by cyclic voltammetry (CV). The secondary batteriesinclude negative electrodes containing different amounts of acetyleneblack as conductive additives.

In this example, Sample M, Comparative Sample N, and Comparative SampleO were prepared.

Sample M was formed in the following manner. With the use of a negativeelectrode active material, a binder, and a disperse medium, a negativeelectrode paste was formed. The negative electrode paste was applied onto the negative electrode current collector and dried. Thus, a negativeelectrode including a negative electrode active material layer wasformed.

In this example, mesocarbon microbeads with a particle diameter of 9 μmwere used as the negative electrode active material, and PVDF was usedas the binder. Mesocarbon microbeads and PVDF were mixed in a ratio of90:10. As the disperse medium for viscosity adjustment, NMP was added toand mixed with the mixture. Thus, the negative electrode paste wasformed.

The negative electrode paste formed by the above method was applied tothe negative electrode current collector (18-μm-thick copper) and driedat 70° C. in an air atmosphere for 30 minutes, and then dried at 170° C.in a reduced pressure environment for 10 hours, whereby the negativeelectrode active material layer was formed. Note that in the negativeelectrode, the amount of mesocarbon microbeads was 9.09 mg/cm². Then,the negative electrode was stamped out into a round shape. The negativeelectrode active material layer had a thickness of approximately 100 μm.

Comparative Sample N and Comparative Sample O were each formed in thefollowing manner. Acetylene black as a conductive additive was added toa negative electrode active material, a binder, and a disperse medium,so that a negative electrode paste was formed. The negative electrodepaste was applied on to a negative electrode current collector anddried. Thus, a negative electrode including a negative electrode activematerial layer was formed.

In the negative electrode paste used for Comparative Sample N,mesocarbon microbeads with a particle diameter of 9 μm were used as thenegative electrode active material, and PVDF was used as the hinder. Inaddition, acetylene black (AB) was used as the conductive additive,Mesocarbon microbeads, and PVDF were mixed in a ratio of 93:2:5. As adisperse medium for viscosity adjustment, NMP was added to and mixedwith the mixture. Thus, the negative electrode paste was formed.

The negative electrode paste formed by the above method was applied tothe negative electrode current collector (18-μm-thick copper) and driedat 70° C. in an air atmosphere for 30 minutes, and then dried at 170° C.in a reduced pressure environment for 10 hours, whereby the negativeelectrode active material layer was formed. Note 2.5 that in thenegative electrode, the amount of mesocarbon microbeads and AB was 10.55mg/cm². Then, the negative electrode was stamped out into a round shape.The negative electrode active material layer had a thickness ofapproximately 100 μm.

In addition, the content ratio of mesocarbon microbeads to AB and PVDFin the negative electrode paste used in Comparative Sample O wasdifferent from that in the negative electrode paste used in ComparativeSample N. Mesocarbon microbeads, AB, and PVDF were mixed in a ratio of0:50.3:49.7. As the disperse medium for viscosity adjustment, NMP wasadded to and mixed with the mixture. Thus, the negative electrode pastewas formed.

The negative electrode paste formed by the above method was applied tothe negative electrode current collector (18-μm-thick copper) and driedat 70° C. in an air atmosphere for 30 minutes, and then dried at 170° C.in a reduced pressure environment for 10 hours, whereby the negativeelectrode active material layer was formed. Note that the amount of ABin the negative electrode was 1.84 mg/cm². Then, the negative electrodewas stamped out into a round shape. The negative electrode activematerial layer had a thickness of approximately 45 μm.

Sample M, Comparative Sample N, and Comparative Sample O were subjectedto CV measurement. A three-electrode cell was used in the measurement.Sample M was used as a working electrode; a lithium metal (metalliclithium immersed in 0.5 M LiTFSA/PP13-FSA), a reference electrode; and aPt mesh, a counter electrode. As an electrolyte solution, LiTFSA,dissolved in P13-FSA at a concentration of 1 mol/L was used. The scanrate was 0.1 mV/sec; the scan range, a spontaneous potential to 2.5 V(vs. Li/Li⁺); the measurement temperature, 25° C.; and the number ofcycles, 3. Note that in the first cycle, scanning was started from anopen-circuit potential.

FIGS. 9A and 9B show the cyclic voltammograms of the first cycle in theCV measurement. FIG. 9A shows the measurement results at currentdensities [A/g] ranging from −0.2 to 0.05. FIG. 9B shows the measurementresults within the range of −0.03 to 0.03 in FIG. 9A. In FIGS. 9A and9B, the thickest lines show the measurement result of Sample M, thesecond thickest lines show the measurement result of Comparative SampleN, and the thin lines show the measurement result of Comparative SampleO.

From the results in FIGS. 9A and 9B, the lithium insertion potential ofSample M was approximately 0.25 V or lower. Peaks of the lithiuminsertion potentials of Comparative Sample N and Comparative Sample Owere observed at around 1.0 V to 1.5 V. Since acetylene black used asthe conductive additive has a higher lithium insertion potential thangraphite, FIGS. 9A and 9B indicate that an irreversible reductionreaction at around 1.0 V to 1.5 V in Comparative Sample N andComparative Sample O caused initial irreversible capacity. In contrast,an irreversible reduction reaction at around 1.0 V to 1.5 V was notobserved in the case of Sample M.

The above results show that acetylene black contained in the comparativesamples is a less-graphitized carbon material and has a higher lithiuminsertion potential than mesocarbon microbeads. This indicates that anirreversible reduction reaction at around 1.0 V to 1.5 V caused theinitial irreversible capacity.

REFERENCE EXAMPLE

Since 1-ethyl-1,3-dimethylpyrrolidinium bis(fluorosulfonyl)amide(abbreviation: 3mP12-FSA) and 2-methyl-5-azoniaspiro[4,4]nonanebis(fluorosulfonyl)amide (abbreviation: 2mAS44-FSA) used in Example 1are novel ionic liquids, synthesis methods thereof are described below.

First, a synthesis method of 3mP12-FSA represented by Structural Formula(803) is described.

The synthesis method of 3mP12-FSA is described using Synthesis Scheme(S-1) below.

First, in a nitrogen atmosphere, bromoethane (19.8 g, 182 mmol) wasadded to dehydrated tetrahydrofuran (10 ml) to which1,3-dimethylpyrrolidine (12.0 g, 121 mmol) was added, and the mixturewas heated and refluxed for 24 hours. Next, the solvent was removed byevaporation, and the obtained white residue was recrystallized with theuse of ethanol and ethyl acetate and then dried under reduced pressureat 80° C. for 24 hours, whereby 1-ethyl-1,3-dimethylpyrrolidiniumbromide (12.9 g, 62 mmol) which is a white solid was obtained.

Next, pure water (30 ml) was added to 1-ethyl-4,3-dimethylpyrrolidiniumbromide (12.9 g, 62 mmol) and potassium bis(fluorosulfonyl)amide (10.1g, 46 mmol). This liquid was stirred in the air at room temperature, sothat a two-layer mixture of an aqueous layer and an objective liquid wasobtained. An object was extracted from the aqueous layer of the mixturewith methylene chloride. The extracting solution and the liquid obtainedin the stirring were combined and washed with pure water 6 times. Thesolvent was removed by evaporation and dried under reduced pressure at60° C. for approximately 8 hours, so that1-ethyl-1,3-dimethylpyrrolidinium bis(fluorosulfonyl)amide (16.26 g, 53mmol) was obtained.

The compound obtained through the above steps was identified as1-ethyl-1,3-dimethylpyrrolidinium bis(fluorosulfonyl)amide which is anobject by using a nuclear magnetic resonance (NMR).

NMR data of the obtained compound is shown below.

¹H-NMR(CDCl₃, 400 MHz, 298 K):δ=1.16(dd, J=6.8 Hz, 2.8 Hz, 2H), 1.24(dd,J=6.8 Hz, 2.8 Hz, 1H), 1.35-1.49(m 3M), 1.80-1.98(m, 1H), 2.32-2.50(m,1H), 2.58-2.76(m, 1H), 2.94-3.15(m, 4H), 3.32-3.83(m, 5H).

Next, a synthesis method of 2mAS44-FSA represented by Structural Formula(805) is described.

The synthesis method of 2mAS44-FSA is described using Synthesis Scheme(S-2) below.

<Synthesis of 3-methylpyrrolidine>

In a 1000 ml recovery flask, 4-amino-2-methyl-1-butanol expressed byStructural Formula (α-1) (52.0 g, 0.5 mol) and carbon tetrachloride(154.0 g, 1.0 mol) were added to methylene chloride (150 ml).Tripheitylphosphine (144.0 g, 0.55 mol) was added to this solution, andthen the mixture was heated to reflux at 40° C. (under atmosphericpressure and a stream of nitrogen) for approximately 3 hours. Then, anobject was extracted from an organic layer with pure water (50 ml) 3times, and the water was removed by evaporation under reduced pressure.The residue was transferred to a 100 ml round bottom flask formed of afluororesin, a sodium hydroxide solution formed by dissolving sodiumhydroxide (40.0 g, 1.0 mol) in 60 ml of water was added to the flask,and the mixture was stirred at room temperature in the air overnight(for 15 hours to 20 hours). After the stirring, NaCl was added to themixture. The mixture was distilled. Thus, 3-methylpyrrolidine (40.64 g,0.48 mol) represented by Structural Formula (α-2) was obtained.

<Synthesis of 2-methyl-5-azoniaspiro[4,4]nonane bromide>

In a 1000 ml three-neck flask, dibromobutane (108.0 g, 0.5 mol) wasadded to a sodium hydroxide solution formed by dissolving sodiumhydroxide (20.0 g, 0.5 mol) in 500 ml of water. The synthesized3-methylpyrrolidine (40.64 g, 0.48 mol) was dripped for 30 minutes withthis mixture kept at 60° C. and stirred, and then the mixture was heatedto reflux at 100° C. in the air for 3 hours. After the reflux, a solventof the mixture was removed by evaporation under reduced pressure,whereby a solid was obtained. A sodium hydroxide solution formed bydissolving sodium hydroxide (100.0 g, 2.5 mol) in 250 ml of water wasadded to the solid under cooling with ice. An object was extracted froman aqueous layer of the two-layer mixture with methylene chloride, andthe obtained extracting solution and organic layer were dried withmagnesium sulfate. The mixture was subjected to gravity filtration, andthe obtained filtrate was concentrated to give a solid. The solid wasrecrystallized with the use of methylene chloride and acetone and then,was washed with diethyl ether. Thus,2-methyl-5-azoniaspiro[4,4]nonanebromide (0.225 mol, 49.6 g) expressedby Structural Formula (α-3) was obtained.

<Synthesis of 2mAS44-FSA>

In a 300 ml conical flask, 2-methyl-5-azoniaspiro[4,4]nonanebromide(50.0 g, 0.225 mol), potassium bis(fluorosulfortyl)amide (54.0 g, 0.248mol), and 30 ml of water were added. This liquid was mixed and stirredin the air at room temperature, so that a two-layer mixture of anaqueous layer and an objective liquid was obtained. An object wasextracted from the aqueous layer of the mixture with methylene chloride.The extracting solution and the liquid obtained in the stirring werecombined, washed with pure water 6 times, and dried with magnesiumsulfate. The mixture was gravity filtered, and the obtained filtrate wasconcentrated to give a liquid. The liquid was dried under reducedpressure at 60° C. for 8 hours to obtain2-methyl-5-azoniaspiro[4,4]nonane bis(fluorosulfonyl)amide (64.11 g,0.20 mol).

The compound synthesized through the above steps was identified as2mAS44-FSA which is an object by using a nuclear magnetic resonance(NMR).

¹H NMR data of the obtained compound is shown below.

¹H-NMR(CDCl₃, 400 MHz, 298 K):δ=1.16-1.25(m, 3H), 1.85-1.95(m, 1H),2.20-2.31(m, 4H), 2.39-2.49(m, 1H), 2.62-2.74(m, 1H), 3.11-3.16(m, 1H),3.45-3.74(m, 7H).

This application is based on Japanese Patent Application serial No.2012-224603 filed with Japan Patent Office on Oct. 9, 2012, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A method for manufacturing a power storage devicecomprising the steps of: mixing a carbon material and binder with asolvent to form a paste; applying the paste on a current collector;drying the paste on the current collector to form a negative electrode;and impregnating a positive electrode, the negative electrode, and aseparator with an electrolyte solution containing a lithium ion and anionic liquid composed of an organic cation and an anion, wherein amelting point of the ionic liquid is −10° C. or lower, wherein a contentpercentage of the carbon material with the R value of 1.1 or more isless than 2 wt % in the negative electrode, wherein the R value is aratio of a peak intensity I₁₃₆₀ to a peak intensity I₁₅₈₀ (I₁₃₆₀/I₁₅₈₀),the peak intensity I₁₃₆₀ and the peak intensity I₁₅₈₀ are observed byRaman spectrometry at a Raman shift of 1360 cm⁻¹ and a Raman shift of 1580 cm⁻¹, respectively.
 3. The power storage device according to claim2, wherein the organic cation is a quaternary ammonium cation, atertiary sulfonium cation, a quaternary phosphonium cation, animidazolium cation, or a pyridinium cation.
 4. The power storage deviceaccording to claim 2, wherein the ionic liquid is represented by aGeneral Formula (G1),

wherein R¹ to R⁶ separately represent an alkyl group having 1 to 20carbon atoms, a methoxy group, a methoxymethyl group, a methoxyethylgroup, or a hydrogen atom, and wherein A⁻ represents a monovalent amideanion, a monovalent methide anion, a fluorosulfonate anion, aperfluoroalkylsulfonate anion, tetrafluoroborate, perfluoroalkylborate,hexafluorophosphate, or perfluoroalkylphosphate.
 5. The power storagedevice according to claim 2, wherein the ionic liquid is represented bya General Formula (G2),

wherein R⁷ to R¹³ separately represent an alkyl group having 1 to 20carbon atoms, a methoxy group, a methoxymethyl group, a methoxyethylgroup, or a hydrogen atom, and wherein A⁻ represents a monovalent amideanion, a monovalent methide anion, a fluorosulfonate anion, aperfluoroalkylsulfonate anion, tetrafluoroborate, perfluoroalkylborate,hexafluorophosphate, or perfluoroalkylphosphate.
 6. The power storagedevice according to claim 2, wherein the ionic liquid includes twoaliphatic rings and is represented by a General Formula (G3),

wherein n and m are greater than or equal to 1 and less than or equal to3, wherein a is greater than or equal to 0 and less than or equal to 4when n is 1, a is greater than or equal to 0 and less than or equal to 5when n is 2, and α is greater than or equal to 0 and less than or equalto 6 when n is 3, wherein β is greater than or equal to 0 and less thanor equal to 4 when m is 1, β is greater than or equal to 0 and less thanor equal to 5 when m is 2, and β is greater than or equal to 0 and lessthan or equal to 6 when m is 3, wherein “α or β is 0” means that atleast one of the two aliphatic rings is unsubstituted, and a case whereboth α and β are 0 is excluded, wherein X or Y is a substituent which isa straight chain or lateral chain alkyl group having 1 to 4 carbonatoms, a straight chain or lateral chain alkoxy group having 1 to 4carbon atoms, or a straight chain or lateral chain alkoxyalkyl grouphaving 1 to 4 carbon atoms, and wherein A⁻ represents a monovalent amideanion, a monovalent methide anion, a perfluoroalkylsulfonate anion,tetrafluoroborate, perfluoroalkylborate, hexafluorophosphate, orperfluoroalkylphosphate.
 7. A method for manufacturing a power storagedevice comprising the steps of: mixing a carbon material comprisinggraphite and binder with a solvent to form a paste; applying the pasteon a current collector; drying the paste on the current collector toform a negative electrode; and impregnating a positive electrode, thenegative electrode, and a separator with an electrolyte solutioncontaining a lithium ion and an ionic liquid composed of an organiccation and an anion, wherein a content percentage of the carbon materialwith the R value of 1.1 or more is less than 2 wt % in the negativeelectrode, wherein the R value is a ratio of a peak intensity I₁₃₆₀ to apeak intensity I₁₅₈₀ (I₁₃₆₀/I₁₅₈₀), the peak intensity I₁₃₆₀ and thepeak intensity I₁₅₈₀ are observed by Raman spectrometry at a Raman shiftof 1360 cm⁻¹ and a Raman shift of 1580 cm⁻¹, respectively.
 8. The powerstorage device according to claim 7, wherein the organic cation is aquaternary ammonium cation, a tertiary sulfonium cation, a quaternaryphosphonium cation, an imidazolium cation, or a pyridinium cation. 9.The power storage device according to claim 7, wherein the ionic liquidis represented by a General Formula (G1),

wherein R¹ to R⁶ separately represent an alkyl group having 1 to 20carbon atoms, a methoxy group, a methoxymethyl group, a methoxyethylgroup, or a hydrogen atom, and wherein A⁻ represents a monovalent amideanion, a monovalent methide anion, a fluorosulfonate anion, aperfluoroalkylsulfonate anion, tetrafluoroborate, perfluoroalkylborate,hexafluorophosphate, or perfluoroalkylphosphate.
 10. The power storagedevice according to claim 7, wherein the ionic liquid is represented bya General Formula (G2),

wherein R⁷ to R¹³ separately represent an alkyl group having 1 to 20carbon atoms, a methoxy group, a methoxymethyl group, a methoxyethylgroup, or a hydrogen atom, and wherein A⁻ represents a monovalent amideanion, a monovalent methide anion, a fluorosulfonate anion, aperfluoroalkylsulfonate anion, tetrafluoroborate, perfluoroalkylborate,hexafluorophosphate, or perfluoroalkylphosphate.
 11. The power storagedevice according to claim 7, wherein the ionic liquid includes twoaliphatic rings and is represented by a General Formula (G3),

wherein n and m are greater than or equal to 1 and less than or equal to3, wherein a is greater than or equal to 0 and less than or equal to 4when n is 1, α is greater than or equal to 0 and less than or equal to 5when n is 2, and α is greater than or equal to 0 and less than or equalto 6 when n is 3, wherein β is greater than or equal to 0 and less thanor equal to 4 when m is 1, β is greater than or equal to 0 and less thanor equal to 5 when m is 2, and β is greater than or equal to 0 and lessthan or equal to 6 when m is 3, wherein “αor β is 0” means that at leastone of the two aliphatic rings is unsubstituted, and a case where both aand flare 0 is excluded, wherein X or Y is a substituent which is astraight chain or lateral chain alkyl group having 1 to 4 carbon atoms,a straight chain or lateral chain alkoxy group having 1 to 4 carbonatoms, or a straight chain or lateral chain alkoxyalkyl group having 1to 4 carbon atoms, and wherein A⁻ represents a monovalent amide anion, amonovalent methide anion, a perfluoroalkylsulfonate anion,tetrafluoroborate, perfluoroalkylborate, hexafluorophosphate, orperfluoroalkylphosphate.
 12. A method for manufacturing a power storagedevice comprising the steps of: mixing a carbon material comprisinggraphene and binder with a solvent to form a paste; applying the pasteon a current collector; drying the paste on the current collector toform a negative electrode; and impregnating a positive electrode, thenegative electrode, and a separator with an electrolyte solutioncontaining a lithium ion and an ionic liquid composed of an organiccation and an anion, wherein a content percentage of the carbon materialwith the R value of 1.1 or more is less than 2 wt % in the negativeelectrode, wherein the R value is a ratio of a peak intensity I₁₃₆₀ to apeak intensity I₁₅₈₀ (I₁₃₆₀/I₁₅₈₀), the peak intensity I₁₃₆₀ and thepeak intensity I₁₅₈₀ are observed by Raman spectrometry at a Raman shiftof 1360 cm⁻¹ and a Raman shift of 1580 cm⁻¹, respectively.
 13. The powerstorage device according to claim 12, wherein the organic cation is aquaternary ammonium cation, a tertiary sulfonium cation, a quaternaryphosphonium cation, an imidazolium cation, or a pyridinium cation. 14.The power storage device according to claim 12, wherein the ionic liquidis represented by a General Formula (G1),

wherein R¹ to R⁶ separately represent an alkyl group having 1 to 20carbon atoms, a methoxy group, a methoxymethyl group, a methoxyethylgroup, or a hydrogen atom, and wherein A⁻ represents a monovalent amideanion, a monovalent methide anion, a fluorosulfonate anion, aperfluoroalkylsulfonate anion, tetrafluoroborate, perfluoroalkylborate,hexafluorophosphate, or perfluoroalkylphosphate.
 15. The power storagedevice according to claim 12, wherein the ionic liquid is represented bya General Formula (G2),

wherein R⁷ to R¹³ separately represent an alkyl group having 1 to 20carbon atoms, a methoxy group, a methoxymethyl group, a methoxyethylgroup, or a hydrogen atom, and wherein A⁻ represents a monovalent amideanion, a monovalent methide anion, a fluorosulfonate anion, aperfluoroalkylsulfonate anion, tetrafluoroborate, perfluoroalkylborate,hexafluorophosphate, or perfluoroalkylphosphate.
 16. The power storagedevice according to claim 12, wherein the ionic liquid includes twoaliphatic rings and is represented by a General Formula (G3),

wherein n and m are greater than or equal to 1 and less than or equal to3, wherein α is greater than or equal to 0 and less than or equal to 4when n is 1, α is greater than or equal to 0 and less than or equal to 5when n is 2, and α is greater than or equal to 0 and less than or equalto 6 when n is 3, wherein β is greater than or equal to 0 and less thanor equal to 4 when m is 1, β is greater than or equal to 0 and less thanor equal to 5 when m is 2, and β is greater than or equal to 0 and lessthan or equal to 6 when m is 3, wherein “α or β is 0” means that atleast one of the two aliphatic rings is unsubstituted, and a case whereboth α and β are 0 is excluded, wherein X or Y is a substituent which isa straight chain or lateral chain alkyl group having 1 to 4 carbonatoms, a straight chain or lateral chain alkoxy group having 1 to 4carbon atoms, or a straight chain or lateral chain alkoxyalkyl grouphaving 1 to 4 carbon atoms, and wherein A⁻ represents a monovalent amideanion, a monovalent methide anion, a perfluoroalkylsulfonate anion,tetrafluoroborate, perfluoroalkylborate, hexafluorophosphate, orperfluoroalkylphosphate.